The present invention pertains to methods and apparatus for heating materials with photoelectric radiant energy. In particular, the present invention pertains to an apparatus and method for pyrolyzing silane (SiH.sub.4) and ammonia (NH.sub.3) gases to form silicon (Si) and silicon nitride (Si.sub.3 N.sub.4) powders.
Considerable attention has been directed to silicon nitride due to its intrinsic properties of low thermal expansion and moderate elastic modulus, and ability to withstand thermal shock. Silicon nitrides have potential structural applications as high-temperature ceramics. However, the same physical properties which make silicon nitride an attractive high-temperature ceramic material create difficulty in fabricating silicon nitride into dense and suitable shapes.
Most commercially available silicon nitride powder is produced by nitriding silicon. The process includes nitriding loose silicon powder in a nitrogen atmosphere for long periods at temperatures between 1,200.degree. C. and 1,350.degree. C. The prolonged period of heating is followed by a shorter period of heating at still higher temperatures above the melting temperature of silicon. Temperature control is important in the process yet temperature is difficult to control due to the exothermic nature of the reaction. A solid cake of silicon nitride is formed which must be reduced to a powder by pulverizing and milling. The silicon nitride powder formed by pulverizing and milling is nonuniform in particle size and shape. Pulverizing and milling may also introduce further contaminants into the silicon nitride powder.
Two general methods for forming shaped silicon nitride articles of manufacture include hot pressing and pressureless sintering. In the method known as hot pressing, a silicon nitride powder is hot pressed with 1-5% by weight of yttrium oxide (Y.sub.2 O.sub.3) at 4,000 pounds per square inch (psi) and temperatures between 1,700.degree. C. and 1,800.degree. C. Hot pressing is usually carried out in graphite dies which are coated with nitride slurries to prevent reactions with the graphite and to ease material removal. Hot pressing results in fully dense silicon nitride materials, but is limited to fairly simple shapes and is very expensive.
Pressureless sintering techniques are also used to densify silicon nitride. In typical pressureless sintering processes, powder shapes are prepared by conventional methods such as injection molding allowing shapes of greater complexity. The silicon nitride mold is then subjected to temperatures of 1,400.degree. C. under a nitrogen atmosphere. Pressureless sintering techniques do not result in fully dense silicon nitride materials.
Both hot pressing and pressureless sintering techniques require silicon nitride powders of uniform composition and purity. Such powders of uniform size and purity are not normally obtained under current commercially available processes. Despite recent improvements in processing silicon carbide and silicon nitride powders produced by heated vapor and arc plasma techniques, it has not been possible to densify silicon carbide and silicon nitride to theoretical densities without the use of pressure or additives.
In a recent article, S. C. Danforth, J. H. Flint, W. R. Cannon and J. S. Haggerty report that silicon and silicon nitride powders can be pyrolyzed from silane and ammonia/silane gas mixtures by direct coupling initiated by thermal energy provided by a CO.sub.2 laser heat source. The powders produced from silane with the use of the laser heat source are characterized by a grain size less than 0.1 micrometer, a narrow size range, little agglomeration, spherical shapes, and high chemical and phase purity. In the process described, a laser beam enters a housing through a potassium chloride window and is arrested by a water cooled copper block. The silicon nitride powders were synthesized by introducing reactant gases orthogonally into a laser beam. Power densities for the laser apparatus were in the range of 250 to 760 watts/cm.sup.2 for an unfocused 0.5-centimeter diameter beam. Reactant gases were injected into the laser beam through a 1-millimeter inside diameter stainless steel inlet tube. An inert gas introduced into the reaction vessel coaxially with the reactant gases minimized reaction gas expansion and directed product powders out of the reaction vessel. Cell pressures were maintained in the range of 0.1 to 0.5 atmosphere. Approximately 600 centimeters per minute argon gas was maintained falling past the laser window to keep the window clear. A 10-to-1 mixture of ammonia to silane was employed with a typical flow rate of 110 cc/min. ammonia to 11 cc/min. silane. See S. C. Danforth, et al, "Laser Synthesis of Silicon Nitride Powders," American Institute of Physics, pp. 659 to 663 (1979).
Despite the recent developments in laser synthesis of silicon nitride powders, the experimental techniques of Danforth et al are not acceptable to scaling up to larger quantities. Specifically, the introduction of reactant gases orthogonally into the laser beam results in uneven heating of the gases. Gas molecules in the stream which are in a position close to the laser receive much more radiant energy than gas molecules in a position distal to the laser. Larger gas streams of reactant gases required for commercial production would require an unrealistic increase in the diameter of the laser beam or the addition of more lasers at great cost.
Thus, there is a need for a method and apparatus for the production of silicon nitride powders by laser pyrolysis that is susceptible to commercial use, which will heat a gas stream in a substantially uniform manner.